Digital coding schemes based on source-channel separation principle are sensitive to channel condition. If the channel condition is worse than what a code is designed for, the decoder fails to completely decode a code that has traversed the channel. On the other hand, if the channel condition is better than what the code is designed for, the code fails to provide any performance improvement. Thus, the digital coding scheme suffers from a threshold effect.
One approach is to design the coding scheme to achieve an optimal performance for a given channel condition. However, the resulting code offers performance improvement either when the channel condition is worse or when the channel condition is better, but not both.